Cardiac arrythmias, and atrial fibrillation in particular, persist as common and dangerous medical ailments, especially in the aging population. In patients with normal sinus rhythm, the heart, which is comprised of atrial, ventricular, and excitatory conduction tissue, is electrically excited to beat in a synchronous, patterned fashion. In patients with cardiac arrythmias, abnormal regions of cardiac tissue do not follow the synchronous beating cycle associated with normally conductive tissue as in patients with normal sinus rhythm. Instead, the abnormal regions of cardiac tissue aberrantly conduct to adjacent tissue, thereby disrupting the cardiac cycle into an asynchronous cardiac rhythm. Such abnormal conduction has been previously known to occur at various regions of the heart, such as, for example, in the region of the sino-atrial (SA) node, along the conduction pathways of the atrioventricular (AV) node and the Bundle of His, or in the cardiac muscle tissue forming the walls of the ventricular and atrial cardiac chambers.
Cardiac arrhythmias, including atrial arrhythmias, may be of a multiwavelet reentrant type, characterized by multiple asynchronous loops of electrical impulses that are scattered about the atrial chamber and are often self propagating. Alternatively, or in addition to the multiwavelet reentrant type, cardiac arrhythmias may also have a focal origin, such as when an isolated region of tissue in an atrium fires autonomously in a rapid, repetitive fashion.
Ventricular tachycardia (V-tach or VT) is a tachycardia, or fast heart rhythm that originates in one of the ventricles of the heart. This is a potentially life-threatening arrhythmia because it may lead to ventricular fibrillation and sudden death.
Diagnosis and treatment of cardiac arrythmias include mapping the electrical properties of heart tissue, especially the endocardium and the heart volume, and selectively ablating cardiac tissue by application of energy. Such ablation can cease or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process destroys the unwanted electrical pathways by formation of non-conducting lesions. Examples of catheter-based devices and treatment methods have generally targeted atrial segmentation with ablation catheter devices and methods adapted to form linear or curvilinear lesions in the wall tissue which defines the atrial chambers, such as those disclosed in U.S. Pat. No. 5,617,854 to Munsif, U.S. Pat. No. 4,898,591 to Jang, et al., U.S. Pat. No. 5,487,385 to Avitall, and U.S. Pat. No. 5,582,609 to Swanson, the disclosures of which are incorporated herein by reference. In addition, various energy delivery modalities have been disclosed for forming such atrial wall lesions, and include use of microwave, laser and more commonly, radiofrequency energies to create conduction blocks along the cardiac tissue wall, as disclosed in WO 93/20767 to Stem, et al., U.S. Pat. No. 5,104,393 to Isner, et al. and U.S. Pat. No. 5,575,766 to Swartz, et al., respectively, the entire disclosures of which are incorporated herein by reference.
In this two-step procedure—mapping followed by ablation—electrical activity at points within the heart is typically sensed and measured by advancing a catheter containing one or more electrical sensors into the heart, and acquiring data at a multiplicity of points. These data are then utilized to select the endocardial target areas at which ablation is to be performed. However, recent techniques have looked to epicardial mapping and ablation to treat ventricular tachycardia. The technique involves introducing a standard ablation catheter into the pericardial space using a subxiphoid pericardial puncture technique.
The parietal pericardium is the outer protective layer or sac that encloses the heart which comprises three layers: epicardium, myocardium and endocardium. A pericardial cavity or space separates the parietal pericardium and the epicardium. A small amount of fluid is secreted by tissues of the parietal pericardium to lubricate surfaces so that heart can move freely inside the parietal pericardium. Clearly, adhesion between the parietal pericardium and the epicardium would interfere with muscular contractions of the heart.
Another potential complication in accessing the epicardium is posed by the phrenic nerve. The phrenic nerve is made up mostly of motor nerve fibers for producing contractions of the diaphragm. In addition, it provides sensory innervation for many components of the mediastinum and pleura, as well as the upper abdomen, especially the liver, and the gall bladder. The right phrenic nerve passes over the right atrium and the left phrenic nerve passes over the left ventricle and pierces the diaphragm separately. Both these nerves supply motor fibers to the diaphragm and sensory fibres to the fibrous pericardium, mediastinal pleura and diaphragmatic peritoneum. Any damage to the phrenic nerve, particularly for senior patients, can cause serious breathing difficulties, especially if the damage is permanent. The lung itself is another organ that is susceptible to damage when ablating the epicardium, although the tissue of the lung can more readily repair itself if burned.
Catheters developed for endocardial uses generally have omnidirectional ablation tips supported on flexible shafts. While such catheters are particularly useful for mapping and ablating in cavities and other tubular regions of or near the heart, the omnidirectional ablation tips when used on the epicardium can significantly increase the risk of harmful and unwanted ablation, such as of the parietal pericardium, the phrenic nerve and/or the lungs. Moreover, the flexible shafts on which such omnidirectional ablation tips are mounted provide no traction or support against a lubricated epicardial surface. The shafts often flip and slide inside the pericardial cavity.
Catheters having lasso assemblies are also known. Such catheters are disclosed in, for example, U.S. Pat. Nos. 6,728,455, 6,973,339, 7,003,342, 7,142,903, and 7,412,273, the entire disclosures of which are hereby incorporated by reference. “Lasso” catheters are particularly useful for circumferential ablations around the ostium of the pulmonary veins as the “lasso” typically is mounted transversely on the catheter so that the lasso can sit on the ostium. Such an orientation however is not suitable for a relatively narrow and flat space such as the pericardial cavity.
Accordingly, it is desirable that a catheter be adapted for the epicardium such that the ablation tip is directional and that the shaft supports tissue contact at the ablation tip and allows a user more control and predictability in the positioning of the ablation tip. To that end, it is desirable that the shaft be stabilized against the epicardium and deflectable off-plane so that the ablation tip can sweep the surface of the epicardium within the confines of the pericardial space with minimal risk of tissue trauma. It is further desirable that the catheter provides continuous feedback of the potential recordings or electrograms (ECGs) inside during ablation so as to allow a user to know whether the undesired potentials have been successfully blocked by the epicardial ablation.